[RFC,01/22] arch_topology: Make register_cpu_capacity_sysctl() tolerant to late CPUs

From: James Morse <james.morse@arm.com>

register_cpu_capacity_sysctl() adds a property to sysfs that describes
the CPUs capacity. This is done from a subsys_initcall() that assumes
all possible CPUs are registered.

With CPU hotplug, possible CPUs aren't registered until they become
present, (or for arm64 enabled). This leads to messages during boot:
| register_cpu_capacity_sysctl: too early to get CPU1 device!
and once these CPUs are added to the system, the file is missing.

Move this to a cpuhp callback, so that the file is created once
CPUs are brought online. This covers CPUs that are added late by
mechanisms like hotplug.
One observable difference is the file is now missing for offline CPUs.

Signed-off-by: James Morse <james.morse@arm.com>
---
If the offline CPUs thing is a problem for the tools that consume
this value, we'd need to move cpu_capacity to be part of cpu.c's
common_cpu_attr_groups.
---
 drivers/base/arch_topology.c | 38 ++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

On 11/7/23 20:29, Russell King (Oracle) wrote:
> From: James Morse <james.morse@arm.com>
> 
> register_cpu_capacity_sysctl() adds a property to sysfs that describes
> the CPUs capacity. This is done from a subsys_initcall() that assumes
> all possible CPUs are registered.
> 
> With CPU hotplug, possible CPUs aren't registered until they become
> present, (or for arm64 enabled). This leads to messages during boot:
> | register_cpu_capacity_sysctl: too early to get CPU1 device!
> and once these CPUs are added to the system, the file is missing.
> 
> Move this to a cpuhp callback, so that the file is created once
> CPUs are brought online. This covers CPUs that are added late by
> mechanisms like hotplug.
> One observable difference is the file is now missing for offline CPUs.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> If the offline CPUs thing is a problem for the tools that consume
> this value, we'd need to move cpu_capacity to be part of cpu.c's
> common_cpu_attr_groups.
> ---
>   drivers/base/arch_topology.c | 38 ++++++++++++++++++++++++------------
>   1 file changed, 26 insertions(+), 12 deletions(-)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>

On Tue, 07 Nov 2023 10:29:23 +0000
Russell King <rmk+kernel@armlinux.org.uk> wrote:

> From: James Morse <james.morse@arm.com>
> 
> register_cpu_capacity_sysctl() adds a property to sysfs that describes
> the CPUs capacity. This is done from a subsys_initcall() that assumes
> all possible CPUs are registered.
> 
> With CPU hotplug, possible CPUs aren't registered until they become
> present, (or for arm64 enabled). This leads to messages during boot:
> | register_cpu_capacity_sysctl: too early to get CPU1 device!
> and once these CPUs are added to the system, the file is missing.
> 
> Move this to a cpuhp callback, so that the file is created once
> CPUs are brought online. This covers CPUs that are added late by
> mechanisms like hotplug.
> One observable difference is the file is now missing for offline CPUs.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> If the offline CPUs thing is a problem for the tools that consume
> this value, we'd need to move cpu_capacity to be part of cpu.c's
> common_cpu_attr_groups.

I'm not keen on squirting sysfs files in from code so
might be nice to do that anyway and use is_visible() / sysfs_update_group()
but that would be a job for another day if at all.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

On Tue, Nov 28, 2023 at 02:37:22PM +0000, Jonathan Cameron wrote:
> On Tue, 07 Nov 2023 10:29:23 +0000
> Russell King <rmk+kernel@armlinux.org.uk> wrote:
> 
> > From: James Morse <james.morse@arm.com>
> > 
> > register_cpu_capacity_sysctl() adds a property to sysfs that describes
> > the CPUs capacity. This is done from a subsys_initcall() that assumes
> > all possible CPUs are registered.
> > 
> > With CPU hotplug, possible CPUs aren't registered until they become
> > present, (or for arm64 enabled). This leads to messages during boot:
> > | register_cpu_capacity_sysctl: too early to get CPU1 device!
> > and once these CPUs are added to the system, the file is missing.
> > 
> > Move this to a cpuhp callback, so that the file is created once
> > CPUs are brought online. This covers CPUs that are added late by
> > mechanisms like hotplug.
> > One observable difference is the file is now missing for offline CPUs.
> > 
> > Signed-off-by: James Morse <james.morse@arm.com>
> > ---
> > If the offline CPUs thing is a problem for the tools that consume
> > this value, we'd need to move cpu_capacity to be part of cpu.c's
> > common_cpu_attr_groups.
> 
> I'm not keen on squirting sysfs files in from code so
> might be nice to do that anyway and use is_visible() / sysfs_update_group()
> but that would be a job for another day if at all.

I'm doing my best, but it's really not helped by the dysfunctional
nature of some parts of the kernel community. I have now decided that
this is not possible to implement. So while it's a nice idea, I don't
think we'll ever see it.

As I mentioned on the 14th November, complete with a patch (and got no
response from anyone):
> Looking into doing this, the easy bit is adding the attribute group
> with an appropriate .is_visible dependent on cpu_present(), but we
> need to be able to call sysfs_update_groups() when the state of the
> .is_visible() changes.
>
> Given the comment in sysfs_update_groups() about "if an error occurs",
> rather than making this part of common_cpu_attr_groups, would it be
> better that it's part of its own set of groups, thus limiting the
> damage from a possible error? I suspect, however, that any error at
> that point means that the system is rather fatally wounded.
>
> This is what I have so far to implement your idea, less the necessary
> sysfs_update_groups() call when we need to change the visibility of
> the attributes.